1. Field of the Invention
The present invention relates to a radiometer and, more particularly, to a radiometer with a correction device for observing reflections or radiations of infrared rays or visible rays from a subject of observation.
2. Description of the Prior Art
A radiometer to be mounted on a flying body, e.g., an airplane or a satellite is disclosed in, for example, J. M. Maisonneuve and M. Dinguirard "SPOT IN-FLIGHT CALIBRATION", 14th CONGRESS OF THE INTERNATIONAL SOCIETY FOR PHOTOGRAMMETERY and J. P. Midan "THE SPOT-HRV (High Resolution Video) INSTRUMENT: AN OVERVIEW OF DESIGN AND PERFORMANCE", IFA 83-109 (1983).
The above-described type of radiometer is generally made up of a radiometer body mounted on the flying body and including a charge-coupled device (CCD) array or similar monodimensional array of photoelectric transducer means, and optics for conducting radiations/reflections from a subject of observation (e.g. land surface) lying in a predetermined width in a direction perpendicular to the direction of flight of the flying body and including a plane mirror. Since radiations/reflections from the subject of observation are extremely susceptible to meteorological conditions, a prerequisite with the observation is to constantly correct the sensitivity of the radiometer body to radiations/reflections in matching relation to the varying meteorological conditions.
One of conventional approaches for the above-mentioned correction is to locate a light source in the form of a lamp having a reference luminance in close proximity to the plane mirror. While ordinary observation is under way, the mirror held in a position optimal for conducting radiations/reflections to the photoelectric transducer is so driven as to conduct light issuing from the lamp to the transducer. After the correction, the mirror is restored to the original position.
Another conventional approach is to use sunlight in place of the light from the lamp. Still another conventional approach is to receive radiations/reflections from a particular range of the subject of observation (i.e. land surface) as a reference level.
The problem with the lamp scheme is that optics including lenses for defining the path for the reference light increases the overall size and weight of the radiometer. Another problem with this kind of scheme is that since the reference light path is offset in a plane perpendicular to the direction of light propagation of the optics, the reflection characteristic of the optics spaced apart from the reference light path or the change in the characteristic cannot be detected. For the same reason, the sunlight scheme cannot detect changes in the reflection characteristic or the transmission characteristic of part of the optics. In addition, the sunlight scheme has a drawback that when the photoelectric transducer is implemented by a linear array sensor or similar sensor having a broad visual field, changes in the sensitivity cannot be detected on a light-sensitive element basis since the apparent angle of sunlight is small (visual diameter of about 0.5 degree).
The conventional approach using radiations/reflections from a particular range as a reference input level as previously stated is free from the deviation of the reference optical path in the optics. However, this kind of scheme involves heavy limitations since a range where the radiation intensity is uniform is limited on the land surface.